With a theatrical flourish befitting a firm that aims to marry Hollywood and Silicon Valley, a small start-up company with some very big backers Thursday demonstrated a living-room entertainment system that it predicts will eventually become as pervasive as the VCR. The system developed by 3DO Co. resembles a souped-up video game machine, but it handles three-dimensional graphics and highly realistic, fast-moving animated images that look like real Hollywood special effects.